Radon is a naturally occurring, water soluble, radioactive gas that evolves in the soil from the breakdown of heavier radioactive elements. Underground well water can transport the radon from the soil into the house, such as when taking a shower, doing laundry, washing dishes, or cooking with tap water. The Environmental Protection Agency (EPA) estimates that it takes about 10,000 picocuries per liter (pCi/L) of radon in water to contribute 1.0 pCi/L of radon in air throughout the house. The ratio of radon in water to radon in bathroom air while showering can be much higher, typically from 100 to 1; to about 300 to 1. The level of radon activity recommended by the EPA is only 4.0 pCi/L and is an "action level" based upon the correlation between radon exposure and lung cancer. Scientists believe radon exposure is the second leading cause of lung cancer and, in retrospect, it is now believed that many cases of "miner's disease" were cancers due to radon exposure.
When radon decays, it emits alpha particles. These are small, heavy, electrically charged, sub-atomic particles consisting of two protons and two neutrons. If an alpha particle strikes the chromosomes in a lung cell, it may alter the way that cell reproduces and, over time, develop into a recognizable cancerous growth.
Since radon is a sparingly soluble gas, it may be removed by gas/liquid separation processes. One method of gas/liquid separation is aeration. Aeration has traditionally been used in water treatment for degassification and odor removal. This process works by increasing the gas-liquid interface, allowing the dissolved gas to diffuse into the gas phase, and then removing the gas from the system. Aeration systems usually take one of three general forms: spray jet aeration, packed tower aeration, or multistaged bubble aeration.
Most prior art techniques for gas/liquid separation require that process flow be depressurized in order remove the radon from the liquid and then repressurized after stripping. Hence, these systems represent substantial expenses in terms of capital and operational costs. Additionally, there are situations where it would be impractical if not impossible to depressurize and then repressurize the process flow. The reason for this is that most prior art systems employing gas to strip radon require a gas/liquid ratio of about 20 to 1. This high gas/liquid ratio requires that the water flow stream be depressurized for radon stripping and then repressurized after stripping, which is labor intensive and expensive.
Another method of water purification is presented in U.S. Pat. No. 5,393,417 issued to Cox which discloses a water remediation and purification apparatus which includes a Venturi having a variably adjustable throat, sensor mechanism and controller which function to maximize the desired level of ozone produced by aggressive cavitation of a contaminated water stream. This apparatus is used to purify water in a process flow by producing free radicals which interact with the contaminants in the water and oxidize them. Thus, Cox discloses the use of a Venturi in a water stream in order to produce ozone to neutralize a multitude of impurities. As radon is a noble gas it can not be neutralized through oxidation process disclosed in Cox because the free radicals produced by the Cox ozone process will not react with radon.
U.S. Pat. No. 5,397,461 issued to Augustin discloses a water treatment system in which a Venturi is employed to thoroughly mix ozone supplied by a generator with water passing through the Venturi. Like the Cox purification system, the system disclosed in Augustin utilizes the introduction of ozone into a water flow to oxidize contaminants. Additionally, the contact chamber which removes the ozone from the water requires that the system be depressurized in order to allow ozone bubbles to expand in size.
There is a need for a compact, mechanically simple gas-stripping system suitable for installation on small to medium wellheads which is capable of removing radon from a pressurized process stream with minimal head loss. There is a further need for a gas-stripping system which can provide 70% radon removal efficiency achieved with an air to liquid volume ratio of less than 0.5 and an overall head loss of less than 25% of wellhead pressure.